Safety method and apparatus for vehicle passengers

ABSTRACT

In a method for protecting passengers in a moving vehicle, radio signals having a predetermined propagation speed and frequency are transmitted from the moving vehicle to an approaching object, such as a second vehicle. Radio signals reflected from the approaching object, and having a different frequency, are received in the moving vehicle. The difference frequency corresponding to the relative speeds between the two vehicles or between the moving vehicle and an object is obtained by utilizing the change in frequency of both signals caused by the Doppler effect, and the level of the reflected signal, corresponding to the distance between the moving vehicle and the approaching object, is concurrently determined. The time remaining before collision of the moving vehicle with the object or another moving vehicle, if the moving vehicle and/or the other vehicle keep advancing, is determined by multiplying the difference frequency by the level of the reflected signal, to produce a signal inflating air bags within such remaining time. The apparatus comprises means for transmitting and receiving radio signals, means for obtaining the difference frequency between the transmitted and reflected signals, a converter operable to convert the difference frequency to an electrical quantity, such as a voltage, and a converter operable to convert the level of the reflected signal to an electrical quantity, such as a second voltage. The apparatus further includes a multiplier to multiply the two electrical quantities to produce actuating signals when the multiplied value exceeds a predetermined value of the remaining time, and includes means for inflating the air bags, responsive to the actuating signals, within the remaining time.

United States Patent 11:] 3,687,213 Sato et al. Aug. 29, 1972 [54]SAFETY METHOD AND APPARATUS ABSTRACT FOR VEHICLE PASSENGERS in a methodfor protecting passengers in a moving 72 Inventors: Kama Sate; reuseHlaatsune, both e radio signals havins amdeterqlined p p s of Toyota,Japan tton speed and frequency are transnutted from the moving vehicleto an approaching object, such as a 1 a Toyota 1 F 0 Klbmhlki secondvehicle. Radio signals reflected from the apm y m p proaching object,and having a difi'erent frequency, are received in the moving vehicle.The difl'erence [22] Sept 1970 frequency corresponding to the relativespeeds PP "L between the two vehicles or between the moving vehicle andan object is obtained by utilin'ng the change in frequency of bothsignals caused by the Doppler ef- Foreign Am feet, and the level of thereflected signal, correspond- Oct. 8, 1969 Japan ..44I79993 ing to thedistance between the moving vehicle and Oct. 8, I969 Japan ..44/79994the pp c i g j i concurrently determined- The time remaining beforecollision of the moving 521 US. (:1. ..ll82, 280/ AB, /98, vehicle withthe Object or Mr moving vehicleif 180/103, 343/7 ED the moving vehicleand/or the other vehicle keep ad- 51 Int. Cl. ..B60r 21/08 "Win81 isdeemimd by mul'iP'ying m [58] Field ofSarch....280/1S0 AB; 180/82, 98,103; Mum? F "5" 9? 343/7 2 produce a signal inflating air wrthm suchremainmg tune. The apparatus comprises means for transmitting andreceiving radio signals, means for obtain- [56] Ram Cited ing thedifference frequency between the transmitted UNITED STATES PHI-Em andreflected signals, a converter operable to convert the differencefrequency to an electrical quantity. 3,420,572 I, such as a yohagg and aconverter operable t0 c nvcrl 3,187,328 6/ i965 Vetter .343]? the levelof the reflected signal to an electrical quanti- 2,974,304 3/l96lNordlund.................180/82 X ty, such as a second voltage. Theapparatus further in- 2,851,120 9/1958 Fogiel ..343/l12 cludes amultiplier to multiply the two electrical quan- 3,442,347 5/1969 Hodgsonet al ..180/98 tities to produce actuating signals when the multiplied3,448,822 6/ 1969 LaLone et al. ..l80/98 value exceeds a predeterminedvalue of the remaining 3,450,414 6/1969 Kobori ..280/l50 time. n inmeans for i fl e i s Primary Examiner-Kenneth H. Betta A ttorney McGlewand Toren responsive to the actuating signals, within the remainingtime.

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INVENTORS RAZUO SATO BY TOMIO HISATSUNE M: arm Town ATToR NEVS SAFETYMETHOD AND APPARATUS FOR VEHICLE PASSENGERS BACKGROUND OF THE INVENTIONIn the event of a collision of a moving vehicle with a stationary objector with another moving vehicle, passengers will be thrown forwardagainst portions of the moving vehicle, due to the inertia of thepassengers, resulting in serious and possibly fatal injuries. To preventsuch injuries, it has been proposed to utilize inflatable air bagsinstalled in a vehicle at locations which a passenger is most likely tocontact, in the event of a collision, and these air bags are inflatedinstantaneously at the moment of the collision to provide a cushion toprotect the passenger from injury.

In one known arrangement, an impact detecting unit is provided on thevehicle to initiate operation of this safety arrangement. In this knownarrangement, gas is supplied to an air bag, to inflate the air bag, onlyat the time when the impact detecting unit is triggered by the shockresulting from the occurrence of a collision. However, the inflation ofthe air bag must be completed before a passenger is brought into directcontact with the vehicle body, because the distance between thepassenger and relatively rigid parts of the vehicle body in front of thepassenger is only several tens of centimeters. This requirement isfulfilled by utilizing a rapid inflation action, such as produced by theexplosion of gunpowder, for example. When the inertia of the passengeris large, however, such rapid inflation of an air bag is likely to causesecondary injury to the passenger.

SUMMARY OF THE INVENTION This invention relates to safety or protectivemeasures for vehicle passengers in the event the vehicle is involved ina collision with either a stationary or a moving object. Moreparticularly, the invention is directed to a method and apparatus forprotecting vehicle passengers from injury and in which the imminence ofa collision is detected in advance of the collision and air bags areinflated before the passenger can contact with the interior of thevehicle body.

In accordance with the invention, a narrow beam signal is transmittedfrom the moving vehicle at a first frequency, and the signal reflectedfrom a stationary or moving object, which the moving vehicle isapproaching, and which has a second frequency, is received in the movingvehicle. The difference freq uency between the two signals,corresponding to the Doppler effect, is determined, and the level of thereflected signal is also determined. The two determined signals,converted into suitable electric quantities, such as voltages, aremultiplied to produce an actuating signal for effecting inflation of airbags within the time period remaining before actual occurrence of thecollision.

The apparatus of the invention further includes means inhibitinginflation of the air bags in the event that the speed of the movingvehicle is very low, and includes other means limiting the maximum valueof the actuating signal. The actuating signal is utilized to rupture adiaphragm separating a container of gas under pressure from the airbags, or may be utilized to trigger active a gas generator to inflatethe air bags.

An object of the invention is to provide a method of protecting vehiclepassengers, in which inflation of an air bag is completed in advance ofan imminent collision and before a passenger is thrown forward by thecollision.

Another object of the invention is to provide such a method in which animminent collision is detected in advance of the collision andcushioning air bags are inflated within a predetermined time periodbefore a collision.

Another object of the invention is to provide apparatus for detecting animminent collision and inflating cushioning air bags, to protectpassengers in a moving vehicle, before the collision actually occurs.

For an understanding of the principles of the invention, reference ismade to the following description of typical embodiments thereof asillustrated in the accompanying drawings:

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a simplified block diagram of apparatus for performing themethod of the invention;

FIG. 2 is a schematic wiring diagram of the apparatus shown in FIG. 1;

FIG. 3 is a plan view illustrating the relation between two vehiclesprior to a collision;

FIG. 4a is a graphic illustration of the relation between thefrequencies of a Doppler signal and the relative speed between twovehicles on a collision course;

FIG. 4b is a graphic illustration of the relation between the level of aDoppler signal and the distance between two vehicles on a collisioncourse;

FIG. 5a is a graphic illustration of measured characteristics ofapproaching vehicles of different kinds, corresponding to FIG. 4b;

FIG. 5b is a graphic illustration of an actually measured Dopplersignal; FIG. 6 is a perspective view illustrating the arrangement of airbags in a vehicle body;

FIGS. '70 and 7b are, respectively, a longitudinal sectional view and atransverse sectional view of one form of gas valve embodying theinvention;

FIG. 8 is a longitudinal sectional view of another form of gas valveembodying the invention; and

FIG. 9 is a longitudinal sectional view of one form of gas generatorembodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS When radio signals, such asaudio frequency or radio frequency waves, having a frequency f, aretransmitted from a vehicle, the relation of this frequency f, with thefrequency f, of radio signals reflected from an approaching vehicle,with the relative speed of the two vehicles being indicated by v, is asfollows:

Since the propagation speed c and tire frequencyf, of the transmittedradio signal are predetermined, the frequency]; of the Doppler signal isproportional to the sum of the speeds of the two vehicles, that is, therelative speed v In the above equation (2), k, is a proportionalconstant and v is the relative speed of approach of the two vehiclestoward each other.

It is generally stated that the level of the Doppler signal is inverselyproportional to the fourth power of the distance when the distancebetween two vehicles is large, and that this level becomes inverselyproportional to the cube or square of the distance as the distancebetween two vehicles decreases. It has been confirmed that the level ofthe Doppler signal is inversely proportional to the distance when thedistance between the vehicles is very small, such as a distance of lessthan m, which is the operational range for which the present inventionis intended.

in other words, the level G of the Doppler signal with relation to thedistance D may be expressed as follows:

where k, is a constant.

In accordance with the present invention, an electrical quantityproportional to the frequency f, of the Doppler signal and an electricalquantity proportional to the lever G of the Doppler signal aredetermined. Then the product S of the two electrical quantities isobtained as follows:

where k is a constant and T is equal to Dlv. Stated another way, 1represents the value obtained by dividing the distance by the relativespeed, and thus indicates the time remaining until an imminent crashoccurs if both vehicles keep moving in the same manner. Thus, if aparticular signal can be produced before the remaining time T isdecreased below a predetermined value, the passenger protectingapparatus can be activated well before the vehicles crash. This meansthe time when the product S exceeds a certain value in equation (4). Atthis time, the particular signal should be developed. it the relativespeed is high, the product S exceeds a predetermined value when thedistance is correspondingly large. In the end, the signal is produced ata predetermined remaining time without relation to the relative speedsof the vehicles.

As may be seen from equation (4), even if the relative speed v is verylow, the product 8 will exceed a predetermined value provided that thedistance D becomes correspondingly smaller. in this case, if a vehicleis involved in an accident, the damage to the vehicle body will be veryslight and substantially no injuries will occur to passengers. Underthis condition, it is not necessary to produce signals, and consequentlyactuation of the safety apparatus is not necessary. Therefore, it isdesirable to provide non-actuating zone characteristics for the firstelectrical quantity proportional to the relative speed of the vehicles,or to provide an upper limit for the second electrical quantity which isinversely proportional to the distance between the vehicles. Thisprevents production of signals when the relative speed is lower than apredetermined value or the distance between the vehicles is smaller thana predetermined value.

Referring to FIG. I, a VHF transmitting, receiving and mixing unit isindicated at A, a Doppler logical operation unit is indicated at B, andan air bag actuating unit is indicated at C. Transmitting, receiving andmixingunitAcomprisesaVl-lFtransmitterLa directional coupler 2, atransmitting and receiving antenna 3, a mixer 4, and an amplifier 5. TheVHF energy emitted from transmitter l is radiated from antenna 3 throughcoupler 2, and antenna 3 has an appropriate directional characteristicand transmits electric waves over a certain beam width. The wavereflected from an approaching object is received by antenna 3 anddirected through coupler 2 to mixer 4, where a part of the output oftransmitter 1 is mixed with the reflected wave. Thereby, there isderived the Doppler signal having the difference frequency between themixed waves, and this Doppler signal is amplified to a desired value byamplifier 5.

Logical operation unit B comprises a frequency-voltage converter 6, adetector 7, a multiplier 8, and a Schmitt circuit 9. The amplifiedDoppler signal is converted, by converter 6, to a DC voltage V, which isproportional to the frequency. Simultaneously, the signal is convertedby detector 7 to a DC voltage V, which is proportional to the level ofthe signal. The two DC voltages V, and V, are multiplied by multiplier 8and, when the output voltage V, of multiplier 8 exceeds a predeterminedvalue in Schmitt circuit 9, respective actuating signals are produced.

Air bag actuating unit C comprises a valve 10, a noninflammable-gascontainer 11 and an air bag 12. Valve 10 is opened by the actuatingsignal from circuit 9, to deliver gas from container 11 into air bag 12to inflate the air bag to prevent impact between a passenger and thevehicle body.

As will be clear from the foregoing description, voltage V, correspondsto the electrical quantity proportional to the frequency j} of theDoppler signal as derived in equation (2), and voltage V, is theelectrical quantity proportional to the level G of the Doppler signal,as derived in equation (3). Consequently, the output voltage V, ofmultiplier 8 is proportional to the product S derived by equation (4).When this output voltage exceeds a predetermined value, this indicatesthat the time remairning before an imminent collision has decreasedbelow a predetermined value. This remaining time is so set as to besufficiently long to permit opening of valve 10 by the signal fromcircuit 9, resulting in inflation of air bag 12. Usually, this time isset at or 200m.

The components 6' and 7' shown in broken lines in the logical operationunit are, respectively, a non-actuating circuit and an upper limitercircuit. When the relative speed between the vehicles is very low,circuit 6' blocks the output V, of converter 6, or reduces it to nearlyzero. Circuit 7' limits the maximum output of detector 7 when thedistance between the vehicles is very small, for example, less than10cm. Thus no signal is provided from circuit 9 when the relative speedis in the range where no physical damage is to be expected from acollision, or in the range where the distance between the vehicles istoo small to measure.

In FIG. 2,which is a detailed schematic wiring diagram of the circuitryshown in FIG. 1, parts corresponding to those in FIG. 1 have beenidentified by the same reference characters. VHF transmitter 1 comprisesan oscillator 21, a multiplier 22 and a coaxial resonator 23. Oscillator21 includes an oscillating transistor 24, resonating capacitors 25 and26, a resonating winding or coil 27, and associated components. Thesignal, with the initial frequency f,, is directed from oscillator 21 tothe idler circuit of mu]- tiplier 22, which multiplier comprises aplurality of coils and capacitors, and high harmonics are produced bymeans of a Varactor diode 28 having non-linear characteristics and whichis provided at the output side of the idler circuit. High harmonics of aspecific order are taken out at the coaxial resonator 23 by means of aresonating circuit comprising a variable capacitor 29 and a winding 30.The high harmonics, thus taken out, are directed into the directionalcoupler 2.

A mixing diode is indicated at 40 and, by utilizing the non-linearcharacteristics of diode 40, the Doppler signal, having the differencefrequency f,, between the output signal of transmitter l and the signalreflected from an approaching vehicle, is obtained. This Doppler signalis amplified by a linear amplifier 5 consisting of an amplifier 51, aninput resistor 52, a negative feedback resistor 53 and a gain regulatingresistor 54. The amplified Doppler signal is fed to the respectiveinputs of detector 7 and converter 6.

Converter 6 comprises a saturation amplifier including an amplifier 60,an input resistor 62, a negative feedback resistor 63 and afrequency-voltage converter, by means of which a DC voltage,proportional to the frequency of the Doppler signal, is obtained. Theoutput of converter 6 is directed into non-actuating zone circuit 6.When the output is higher than a predetermined value, the voltage Vhaving the specified mark and a value corresponding to the frequency ofthe Doppler signal, is obtained. Non-actuating zone circuit 6 comprisesa linear amplifier including an amplifier 64, resistors 65, 68 and 69,and a bias circuit comprising resistors 66 and 67. The width of thenon-actuating cone 6' is determined by adjusting resistor 67 inaccordance with the voltage to be applied to the bias circuit.

The Doppler signal supplied to detector 7 is rectified by a voltagedoubler circuit, comprising capacitors 71 and 74 and diodes 72 and 73,into a DC voltage proportional to the level of the Doppler signal.Component 7 is a saturation amplifier which acts as an upper limiter andwhich comprises an amplifier 75 and resistors 76, 77 and 78. When itsinput voltage is higher than the predetermined value, the output islimited to a constant value.

The DC voltages V, and V, are directed into multiplier 8, which producesa voltage V, proportional to the product of the voltages V, and V,. Byway of example, multiplier 8 may comprise a Hall generator. Voltage V,is directed into Schmitt circuit 9 comprising an amplifier 91 andpositive feedback resistors 92, 93 and 94. When voltage V, exceeds apredetermined value, an output actuating signal is produced anddelivered to actuating circuit which comprises a thyristor 95 and abattery 97, which produce a current flow through a fusible element 96 tofuse the latter. Thyristor 95 is triggered conductive by the outputsignal of Schmitt circuit 9 which is supplied to its gate. When element96 is fused, valve 10 of FIG. 1, for example, is opened and thenon-inflammable gas is delivered into air bag 12 to inflate the air bag.

FIG. 3 illustrates a moving vehicle 100 provided with the transmittingand receiving antenna 3, and illustrates a second vehicle 200 which ismoving either ahead of vehicle 100 or toward vehicle 100. The relativespeed of the two vehicles is indicated by V and the distancetherebetween by D. The beam angle of the wave radiated by antenna 3 isindicated at 0:.

FIGS. 40 and 4b illustrate the characteristics of the voltages V, and V,which change in relation to the mentioned variables. FIG. 4a illustratesvoltage V l which is proportional to the relative speed V, and FIG. 4billustrates the voltage V, which is inversely proportional to thedistance D between the vehicles. As seen from equations (2) and (3),voltages V, and V, are proportional, respectively, to the frequency andto the level of the Doppler signal, and are obtained by the apparatusillustrated in FIGS. 1 and 2.

In FIG. 5a, which illustrates measured values of the voltage V,, curvesas and b show the voltage level of the Doppler signal produced bytransmitted waves reflected from a large truck. Curves c and d show thevoltage level of the Doppler signal in the case of a small passengervehicle, and curve e illustrates the mean value of these four measuredvalues. Curve e nearly agrees with the inversely proportional curve fobtained by selecting appropriately the proportional constant in thepractical range of vehicle distances of l to 5m. FIG. 5b illustrates theactual waveform of the measured Doppler signal.

It will be clear from FIGS. 50 and 5b that the level G decreases nearlyinversely with the increase in the distance D. The range where the levelG is significantly changed with the change in the distance D correspondsto the distance range where the beam angle a becomes narrower than thewidth of an oncoming or approaching vehicle, a shown in FIG. 3.

FIG. 6 illustrates the manner in which air bags are installed in avehicle. As shown in this figure, air bags I21 and 123 are arranged inthe upper and lower portions of the front panel, and a third air bag 122is located on the arm rest of the door. A passenger is indicated at 120.The air bags are inflated as shown in broken lines, to prevent thepassenger from striking against the vehicle body when he is thrownforward.

FIGS. 70 and 7b illustrate one form of gas valve corresponding to thevalve 10 illustrated in FIG. 1. In these figures, a pipe is indicated at131 and a ring of fusible metal, such as a lead-tin alloy, isillustrated at 132. An insulating partition wall 133 is installed withinring 132, and electrodes 134 are provided at the top and bottom of ring132, with an insulator 135 supporting ring 132 in pipe 13]. Upon receiptof an actuating signal from Schmitt circuit 9 of FIG. 1, electriccurrent flows between electrodes 134 to fuse ring 132, as a result ofwhich partition wall 133 is pushed out by the gas pressure to connectthe space A with the space B. Thus, non-inflammable gas is deliveredinto the air bags as gas valve 130 is opened, and inflates the air bagswithin a predetermined period of time which is determined by the timethe actuating signal is produced.

Another form of gas valve is illustrated in FIG. 8, in which valve isprovided between gas container A and pipe B leading to the air bags.Valve 140 comprises an intermediate pipe 141, a partition wall 142, afrustoconical layer of gunpowder 143, a damp-proof coating agent 144, anelectric detonator 145, lead wires 146, an

insulating seal 147 and a coupling 148. The actuating signal from theSchmitt circuit causes an electric current to flow through lead wires 46to ignite detonator 145, thus detonating gunpowder layer 143. Thedetonation of this gunpowder layer effectively destroys partition wall142 and, at the same time, blows off the coating agent 144, thusallowing delivery of high pressure gas from container A to pipe B.

FIG. 9 illustrates one typical embodiment of a gas producer 150 whichcomprises a container 151, gunpowder 152 for generating gas, an electricdetonator 153, an electrode 154, a plug 155, lead wires 156, a seal 157,a dampproof film 158, a pipe 159, and other associated parts. Thisproducer is supplied with electric current through lead wires 156responsive to the actuating signal from the Schmitt circuit, to ignitedetonator 153. Thereby, gunpowder 152 burns and develops a high pressuregas. The damp-proof film 158 is broken by the gas pressure, so that thehigh pressure gas is delivered through the pipe 159 into the air bag(not shown) to inflate the latter rapidly. Since inflation of the airbag is completed just before the vehicle collides, secondary injury tothe passengers is avoided.

Summarizing, in accordance with the invention, actuating signals areproduced within a predetermined period of time before an imminentcollision, to inflate air bags without delay. The actuating signalproducing time is so selected as to assure the necessary time for theinflation of the air bags, without relation to the colliding speed ofthe vehicles. Consequently, inflation of the air bags can be completedbefore a passenger is thrown forward, due to his inertia, thuspreventing impacting of a passenger on the vehicle body withoutincurring any unnecessary injury to the passenger.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:

l. A method of protecting passengers in a moving vehicle, upon collisionof the moving vehicle with a fixed object or with another movingvehicle, said method comprising the steps of transmitting radio signals,having a first frequency, from the moving vehicle; receiving, in themoving vehicle, the reflected radio signals, having a second frequency,from an object in the path of the moving vehicle; producing a firstelectrical quantity proportional to the difference frequency between thetransmitted and reflected radio signals; producing a second electricalquantity proportional to the level of difference frequency; multiplyingsaid electrical quantities to produce an actuating signal when theproduct of said electrical quantities exceeds a predetermined value; andutilizing said actuating signal to activate inflatable passengerprotecting means in advance of the collision of the moving vehicle withan object in its path.

2. A method of protecting passengers in a moving vehicle, as claimed inclaim 1, including the step of providing a non-actuating zonecharacteristic for said first electrical quantity.

3. A method of protecting passengers in a moving vehicle, as claimed inclaim 1, including limiting the f 'dseco d l tri tit E S35; ap g aratusfor pi' te n r i asslngers in a moving vehicle upon collision of themoving vehicle with another vehicle or with an object in its path, saidsafety apparatus comprising, in combination, transmitting means operableto transmit radio signals, having a first frequency, toward an object inthe path of the moving vehicle; receiving means on the moving vehiclereceiving radio signals reflected from the object and having a secondfrequency; mixer means connected to said transmitting and receivingmeans and deriving the difference frequency between the transmitted andreflected signals; converter means connected to said mixer means toprovide a first electrical quantity proportional to said differencefrequency; detector means connected to said mixer means to provide asecond electrical quantity proportional to the level of said differencefrequency; multiplier means connected to said converter means and tosaid detector means to provide a third electrical quantity proportionalto the product of said first and second electrical quantities; actuatingsignal producing means connected to said multiplier means and producingan actuating signal when said third electrical quantity exceeds apredetermined value; passenger protecting inflatable air bags in saidmoving vehicle; and means connected to said actuating signal producingmeans and operable, responsive to said actuating signal, to inflate saidair bags in advance of collision of the moving vehicle with an object inits path.

5. Safety apparatus for protecting passengers in a moving vehicle, asclaimed in claim 4, including means connected between said convertermeans and said multiplier and operable to render said actuating signalproducing means ineffective when the speed of the moving vehicle or thedistance between the moving vehicle and an object in its path are lessthan predetermined values.

6. Apparatus for protecting passengers in a moving vehicle, as claimedin claim 4, including limiting means connected between said detectormeans and said multiplier and limiting said second electrical quantityto a predetermined upper level.

7. Apparatus for protecting passengers in a moving vehicle, as claimedin claim 4, in which said actuating signal producing means is a Schmitttrigger circuit.

8. Apparatus for protecting passengers in a moving vehicle, as claimedin claim 4, in which said air bags are connected to a source of gasunder pressure by a normally closed valve; said actuating signalinitiating rupture of said valve.

9. Apparatus for protecting passengers in a moving vehicle, as claimedin claim 4, including a normally inactive gas generator connected tosaid air bags; said actuating signal effecting activation of said gasgenerator.

10. Apparatus for protecting passengers in a moving vehicle, as claimedin claim 4, in which said radio signals are transmitted in a narrow beamdirected forwardly of the moving vehicle.

* II l i I!

1. A method of protecting passengers in a moving vehicle, upon collisionof the moving vehicle with a fixed object or with another movingvehicle, said method comprising the steps of transmitting radio signals,having a first frequency, from the moving vehicle; receiving, in themoving vehicle, the reflected radio signals, having a second frequency,from an object in the path of the moving vehicle; producing a firstelectrical quantity proportional to the difference frequency between thetransmitted and reflected radio signals; producing a second electricalquantity proportional to the level of difference frequency; multiplyingsaid electrical quantities to produce an actuating signal when theproduct of said electrical quantities exceeds a predetermined value; andutilizing said actuating signal to activate inflatable passengerprotecting means in advance of the collision of the moving vehicle withan object in its path.
 2. A method of protecting passengers in a movingvehicle, as claimed in claim 1, including the step of providing anon-actuating zone characteristic for said first electrical quantity. 3.A method of protecting passengers in a moving vehicle, as claimed inclaim 1, including limiting the upper value of said second electricalquantity.
 4. Safety apparatus for protecting passengers in a movingvehicle upon collision of the moving vehicle with another vehicle orwith an object in its path, said safety apparatus comprising, incombination, transmitting means operable to transmit radio signals,having a first frequency, toward an object in the path of the movingvehicle; receiving means on the moving vehicle receiving radio signalsreflected from the object and having a second frequency; mixer meansconnected to said transmitting and receiving means and deriving thedifference frequency between the transmitted and reflected signals;converter means connected to said mixer means to provide a firstelectrical quantity proportional to said difference frequency; detectormeans connected to said mixer means to provide a second electricalquantity proportional to the level of said difference frequency;multiplier means connected to said converter means and to said detectormeans to provide a third electrical quantity proportional to the productof said first and second electrical quantities; actuating signalproducing means connected to said multiplier means and producing anactuating signal when said third electrical quantity exceeds apredetermined value; passenger protecting inflatable air bags in saidmoving vehicle; and means connected to said actuating signal producingmeans and operable, responsive to said actuating signal, to inflate saidair bags in advance of collision of the moving vehicle with an object inits path.
 5. Safety apparatus for protecting passengers in a movingvehicle, as claimed in claim 4, including means connected between saidconverter means and said multiplier and operable to render saidactuating signal producing means ineffective when the speed of themoving vehicle or the distance between the moving vehicle and an objectin its path are less than predetermined values.
 6. Apparatus forprotecting passengers in a moving vehicle, as claimed in claim 4,including limiting means connected between said detector means and saidmultiplier and limiting said second electrical quantity to apredetermined upper level.
 7. Apparatus for protecting passengers in amoving vehicle, as claimed in claim 4, in which said actuating signalproducing means is a Schmitt trigger circuit.
 8. Apparatus forprotecting passengers in a moving vehicle, as claimed in claim 4, inwhich said air bags are connected to a source of gas under pressure by anormally closed valve; said actuating signal initiating rupture of saidvalve.
 9. Apparatus for protecting passengers in a moving vehicle, asclaimed in claim 4, including a normally inactive gas generatorconnected to said air bags; said actuating signal effecting activationof said gas generator.
 10. Apparatus for protecting passengers in amoving vehicle, as claimed in claim 4, in which said radio signals aretransmitted in a narrow beam directed forwardly of the moving vehicle.